1. Field of the Invention
This invention relates to active control of noise in ducts.
2. Description of Related Art
The principles of active noise control were established by Paul Lueg in 1936 and basically consist of detecting by a microphone the noise which it is wished to control, and replaying the detected noise in anti-phase via a loudspeaker so that the regenerated noise destructively interferes with the source noise. Since that time there has been a great deal of research in the field of active noise control. However, the basic configuration for active noise control in a duct has been the provision of the microphone in the centre of the duct and of the loudspeaker in the duct wall. There are good reasons for this arrangement which will be gone into in greater detail later on in this specification.
However, it has been discovered that this known arrangement has disadvantages when there is a fluid medium flowing through the duct.